1. Field of the Invention
The present invention relates to a plasma processing apparatus for performing a process, such as a film formation process or etching process, by use of plasma on target objects, such as semiconductor wafers, and particularly to a technique utilized in the semiconductor process field. The term “semiconductor process” used herein includes various kinds of processes which are performed to manufacture a semiconductor device or a structure having wiring layers, electrodes, and the like to be connected to a semiconductor device, on a target object, such as a semiconductor wafer or a glass substrate used for an FPD (Flat Panel Display), e.g., an LCD (Liquid Crystal Display), by forming semiconductor layers, insulating layers, and conductive layers in predetermined patters on the target object.
2. Description of the Related Art
In manufacturing semiconductor devices for constituting semiconductor integrated circuits, a target object, such as a semiconductor wafer, is subjected to various processes, such as film formation, etching, oxidation, diffusion, reformation, annealing, and natural oxide film removal. US 2006/0286817 A1 discloses a semiconductor processing method of this kind performed in a vertical heat-processing apparatus (of the so-called batch type). According to this method, semiconductor wafers are first transferred from a wafer cassette onto a vertical wafer boat and supported thereon at intervals in the vertical direction. The wafer cassette can store, e.g., 25 wafers, while the wafer boat can support 30 to 150 wafers. Then, the wafer boat is loaded into a process container from below, and the process container is airtightly closed. Then, a predetermined heat process is performed, while the process conditions, such as process gas flow rate, process pressure, and process temperature, are controlled.
In order to improve the characteristics of semiconductor integrated circuits, it is important to improve the properties of insulating films used for semiconductor devices. Conventionally, SiO2 films are mainly used as insulating films for semiconductor devices. However, in recent years, semiconductor integrated circuits are required to satisfy further improved integration and miniaturization levels. Under the circumstances, silicon nitride films (Si3N4 films) are used as insulating films for oxidation-resistant films, impurity-diffusion prevention films, and/or sidewall films of gate devices. Silicon nitride films are low in impurity diffusion coefficient and high in oxidation barrier property, and thus are extremely suitable for insulating films used in the applications described above.
Further, in recent years, semiconductor integrated circuits are required to have a higher operation speed. Owing to this demand, there is a proposal in which a silicon nitride film doped with an impurity, such as boron B, is formed to provide an insulating film having a far smaller dielectric constant that decreases the parasitic capacitance to a large extent (Jpn. Pat. Appln. KOKAI Publication No. 6-275608).
In addition to the demand described above, it is necessary to lower the temperature of processes. Owing to this demand, there has been proposed a plasma processing apparatus that performs a process by use of plasma to promote a reaction even at a lower wafer temperature (Jpn. Pat. Appln. KOKAI Publications No. 2006-270016 and No. 2007-42823).
FIG. 25 is a view schematically showing a vertical plasma processing apparatus, which is one of the conventional apparatuses described above. FIG. 26 is a sectional view showing part of a plasma box used in the apparatus shown in FIG. 25. As shown in FIG. 25, a cylindrical process container made of quartz and configured to vacuum-exhaust the internal atmosphere is disposed to accommodate semiconductor wafers (not shown) arrayed at intervals in the vertical direction. A plasma generation box 4 having a rectangular shape in a cross section is attached to the sidewall of the process container 2 along the vertical direction. A gas nozzle 5 for supplying a gas to be activated by plasma is disposed inside the box 4. As shown also in FIG. 26, two distinct plasma electrodes 6 are disposed to extend in the vertical direction on opposite sides outside the wall that defines the plasma generation box 4. An RF (radio frequency) power with, e.g., 13.56 MHz for plasma generation is applied across the two plasma electrodes 6 from an RF power supply 8.
With the arrangement described above, the two plasma electrodes 6 render an electrode pair of the parallel-plate type, so that plasma is generated by capacitive coupling when an RF power is applied across the plasma electrodes 6. The plasma thus generated activates a gas supplied into the plasma box 4, thereby generating active species or radicals that promote the reaction and so forth. A plasma processing apparatus of this type is generally called a plasma processing apparatus of the CCP (Capacitively Coupled Plasma) type.
According to a plasma processing apparatus of the CCP type, a reaction for, e.g., film formation is promoted by assistance of plasma, and so a predetermined plasma process can be performed even at a relatively low wafer temperature. However, the present inventors have found that plasma processing apparatuses of this type entail problems concerning particle generation and electron density.